But Kevin Blackwell is biding his time before deciding whether to return with an improved offer.
Leeds face competition for midfielder Nicholls from Wolves, who are understood to have tabled a £400k offer despite the absence of a manager at Molineux, and both Norwich and Cardiff have been linked with moves for striker Howard.
Nicholls and Howard are out of contract next summer and Blackwell is unlikely to be tempted to pay over-the-odds for either player with a limited budget available to him.
Blackwell said in the YEP: "We made an offer which we thought was the right valuation for the players concerned. Luton have turned it down as they have every right to do. We're monitoring the situation."